1. Field of the Invention
The present invention relates to a zoom lens system, an imaging device and a camera, and more particularly, to a small-size and high-image-quality zoom lens system having a viewing angle of 60° to 70° at the wide-angle limit and a zoom ratio of about 3×, which is suitable for digital still cameras, digital video cameras and the like, an imaging device having the zoom lens system, and a camera having the imaging device.
2. Description of the Background Art
In digital still cameras using a solid-state image sensor such as a CCD (charge coupled device) or a CMOS (complementary metal-oxide semiconductor), since a member such as an optical low-pass filter is disposed between the rearmost part of the lens elements and the solid-state image sensor, a lens system having a comparatively long back focal length is required. Moreover, the image-taking optical system of digital still cameras is required of an excellent telecentric characteristic in order to avoid shading which is a factor that decreases the quantity of peripheral light on the image surface.
Many forms of digital still cameras can be considered, and one form is a compact type. As zoom lens systems suitable for compact-type digital still cameras, there have been proposed many three-unit zoom lens systems, in order from the object side, including a first lens unit having negative power, a second lens unit having positive power and a third lens unit having positive power (Japanese Laid-Open Patent Publications Nos. 2002-196240, 2001-242378, 2001-272602 and 2003-287679).
However, in the zoom lens system described in Japanese Laid-Open Patent Publication No. 2002-196240, the second lens unit has many lens elements and therefore, it is difficult to retract the zoom lens system into a compact form when not in use. The zoom lens system described in Japanese Laid-Open Patent Publication No. 2001-242378 has fewer lens elements, but the second lens unit has a large thickness and therefore, it is difficult to retract the zoom lens system into a compact form when not in use. The zoom lens system described in Japanese Laid-Open Patent Publication No. 2001-272602 has fewer lens elements and therefore, can be retracted into a compact form. However, the zoom ratio thereof is as small as about 2×. In Japanese Laid-Open Patent Publication No.2003-287679, there is disclosed a zoom lens system in which the second lens unit includes a cemented lens element having three lens elements which are cemented with each other. However, the second lens unit has a large overall thickness.